Unknown debt and prescribed letter

I am having the worst experience with this company. They work with MTN as their collections department! I Had a contract with mtn that was on 36 months and the term ended without me owning then any outstanding balance to my surprise i started receiving statement that i owe mtn R2100 tried going in store they couldn't resolve the matter i was tolf to contact they're legal team of which till to date i have not received any response from from i have to going up and down since 2022 about this matter now i have being asking nu debt for a prescribed letter and still i have not received anything please help